1. Field of the Invention
The present invention relates to a manufacturing method for an optical fiber grating used in an optical communication system (hereinbelow, referred to simply as an xe2x80x9coptical fiber grating), and in particular, an optical fiber grating, a manufacturing method for the same, and a manufacturing apparatus for the same that allows fine adjustment of the optical properties of the optical fiber grating, and greatly improves the manufacturing yield.
2. Description of the Related Art
An optical fiber grating is a device having the property of attenuating the light of a specific frequency by coupling it in a reflection mode, a cladding mode, or a radiation mode. A type that couples the light of a specific wavelength with the cladding mode or radiation mode in the same direction as the incident light is called a transmission type, and a type that couples with light of a specific wavelength with a waveguide mode (this includes the core mode and reflection mode), cladding mode, or radiation mode in the direction opposite to the incident light is called a reflecting type.
In the spectrum of the reflected light or the transmitted light of the optical fiber grating, a peak is obtained by attenuating light of certain wavelengths, and the size of this peak is called the rejection ratio, the wavelength of its center is called the central wavelength, and the size of the band of the peak is called the rejection bandwidth.
Irrespective of whether the optical fiber grating is of a long-period type or a short-period type, in the case that the optical fiber grating is used as an optical attenuating device such as a gain equalizer, attenuator, band erase filter, or the like, the transmitted light is monitored, and the transmission loss is used as the rejection ratio. In contrast, in the case that the optical fiber grating is used as a reflector (mirror), a wavelength separation device, or the like, the reflected light is monitored, and the reflection rate is used as the rejection ratio.
The optical fiber grating is obtained by forming a grating part having a constant periodic change, for example, a periodic change of the refractive index of the core, in the longitudinal direction of an optical fiber. In order to produce this periodic change in the refractive index, usually ultraviolet light is used. As a method of manufacturing the grating part using ultraviolet light, the following method is generally carried out.
First, the optical fiber is exposed to hydrogen gas (hereinbelow, referred to simply as xe2x80x9chydrogen loadingxe2x80x9d), and the sensitivity of the optical fiber (in particular its core) to fluctuation of the refractive index caused by ultraviolet light is increased. In addition, by irradiating ultraviolet light at a predetermined period along the longitudinal direction of the optical fiber (core), the refractive index of the irradiated part is increased, and the grating part is formed. In the irradiation of the ultraviolet light, methods such as the interference exposure method, the phase mask method, the amplitude mask method, the exposure to a focused beam method, and the like are used, and in each of these methods, the refractive index of the part exposed to the ultraviolet light is increased, and refractive index fluctuation along the longitudinal direction of the optical fiber can be formed.
In addition, there is the case in which the period is a constant interval, and the case in which a chirped pitch, in which the period changes along the longitudinal direction of the optical fiber, is applied. In addition, in order to realize the reflection type, a short period type is necessary, and in order to realize a transmission type, a long period type is necessary.
Moreover, the core of the optical fiber comprises silica glass doped with germanium, and the germanium plays the role of increasing the refractive index of the silica glass by being irradiated with ultraviolet light. Depending on the case, an optical fiber having a part or all of the cladding provided around the core also doped with germanium is used, and the refractive index that changes periodically is formed in the cladding as well. In addition, there is the case in which an optical fiber having only the cladding doped with germanium is used, and a periodic change in the refractive index is formed only in the cladding.
It is known that the refractive index of the silica is increased when doped with germanium, and in the case that the cladding is doped with germanium, in order to adjust the refractive index of the core and cladding, generally doping is carried out using one or several kinds of other dopants.
Aluminum and phosphorus are known as dopants that increase the refractive index of silica glass. In addition, boron and fluorine are known as dopants that act to lower the refractive index. That is, in the case that the cladding is doped with germanium, in order to compensate the increase in the refractive index due to the doping with germanium, boron or fluorine, for example, can be added.
Following the ultraviolet light irradiation processing, preferably, the change in the refractive index of the cladding is suppressed by carrying out dehydrogenation processing, and long term stabilization of the optical properties can be implemented.
Furthermore, preferably as a method for adjusting the loss or reflection of the central wavelength (hereinbelow, referred to as the xe2x80x9ccentral wavelengthxe2x80x9d) and improving the manufacturing yield, heat processing or uniform irradiation in which the cladding as a whole is irradiated by ultraviolet light (hereinbelow, referred to as xe2x80x9cuniform ultraviolet irradiationxe2x80x9d) is carried out.
Specifically, by the uniform ultraviolet irradiation, along with the increasing average refractive index, the central wavelength is shifted to a longer wavelength. In addition, by the heat processing, the refractive index change is made small, which is to say that the average refractive index is made small, and the central wavelength is shifted to a shorter wavelength. Therefore, in the case that the central wavelength of the manufactured optical fiber grating is a shorter wavelength that the desired value, the central wavelength is adjusted by uniform ultraviolet irradiation, and in the case that it is a longer wavelength, the central wavelength is adjusted by carrying out heat processing. In the conventional method, only the central wavelength is given attention, and thus only one of either uniform ultraviolet irradiation or heat processing is carried out.
Finally, heat aging is carried out in order to provide thermal stability.
However, recently in the case that a band rejecting filter or the gain equalizer for an optical amplifier, for example, is formed using optical fiber grating, in addition to a precise central wavelength, a very precise rejection ratio is required.
However, in the conventional method, in the processing of adjusting the central wavelength by heat processing or uniform ultraviolet irradiation, the rejection ratio changes. For example, in the cladding part, the amount of refractive index change caused by excitation by ultraviolet light becomes small due to heating, and as a result, the rejection ratio becomes small.
Therefore, if only the central wavelength is of concern, it can be controlled comparatively stably, but there is no adjustment mechanism for the rejection ratio, and while the optical properties of the product can be stably maintained, there is difficulty in guaranteeing a high yield.
In addition, the change of the refractive index of the optical fiber grating is sensitive to the heat environment (that is, thermal instability) particularly after ultraviolet irradiation, and stably maintaining the stability of the optical properties in this environment over the long term is difficult.
In consideration of the above described problems, it is an object of the present invention to present a method and apparatus that controls the central wavelength and rejection ratio with high precision, and at the same time precisely adjusts and thermally stabilizes the optical properties during manufacture of the optical fiber grating.
In order to resolve the above-described problems, in the present invention, the following resolution device is proposed. According to the optical fiber grating manufacturing method of the present invention, the uniform ultraviolet irradiation processing and the heat processing are carried out in a suitable combination, and thereby, the central wavelength of the optical fiber grating and the rejection ratio at the central wavelength can be adjusted, and an optical fiber grating having the target optical properties can be obtained.
A first aspect of the present invention is a manufacturing method for an optical fiber grating comprising the steps of: forming a grating part having a periodic refractive index distribution by irradiating an optical fiber along the longitudinal direction by ultraviolet light at a predetermined period and carrying out dehydrogenation when necessary; carrying out at least one time uniform ultraviolet irradiation processing that irradiates the grating part as a whole with ultraviolet light; and carrying out heat aging in order to stabilize the optical properties of the grating part.
Moreover, before the grating part formation processing, preferably a hydrogen loading treatment is carried out, and in the case that hydrogen loading treatment is carried out, dehydrogenation processing is preferably carried out before the uniform ultraviolet irradiation processing.
A second aspect of the present invention is a manufacturing method for an optical fiber grating according to the first aspect wherein, before or after the uniform ultraviolet irradiation processing, heat trimming processing is carried out at least one time by heating the grating as a whole in order to adjust the optical properties.
A third aspect of the present invention is a manufacturing method for an optical fiber grating according to aspects 1 or 2 wherein the uniform ultraviolet irradiation processing and the heat trimming processing are repeatedly carried out an arbitrary number of times and in an arbitrary sequence.
A fourth aspect of the present invention is a manufacturing method for an optical fiber grating according to aspects 1, 2, or 3 wherein the uniform ultraviolet irradiation processing and the heat trimming processing are carried out while monitoring the transmitted light, the reflected light, and the reference light of the optical fiber.
A fifth aspect of the present invention is an optical fiber grating comprising a grating part having a periodic refractive index distribution due to irradiation of the optical fiber along the longitudinal direction by ultraviolet light at a predetermined period and a sample fiber having a constant refractive index wherein: the minimum refractive index of the grating part is larger than the refractive index of the sample fiber, and the variation of the smallest refractive index of the grating part is sufficiently smaller than the amount of change in the periodic refractive index.
A sixth aspect of the present invention is a manufacturing apparatus for an optical fiber grating providing an ultraviolet irradiating apparatus and a heating apparatus for adjusting the optical properties of the optical fiber grating.
A seventh aspect of the present invention is a manufacturing apparatus for an optical fiber grating according to aspect 6 wherein a mechanism that implements the uniform ultraviolet irradiation processing and the heat trimming processing maintains a constant tension on the optical fiber.
According to the optical fiber grating apparatus of the present invention, the optical properties of the optical fiber grating are measured, and depending on the results of this measurement, suitable ultraviolet irradiation processing and heat processing can be easily carried out. In addition, the tension of the optical fiber can be maintained at a suitable constant by a tension maintaining mechanism provided on the fiber clamp, and thereby modulation or fluctuation of the optical propertied due to insufficient or excess tension or changes in the tension can be prevented, and precise control of the optical properties becomes possible.